Folding of long DNA chains in the presence of distearyldimethylammonium bromide and unfolding induced by neutral liposomes

Abstract

The interaction between large duplex T4DNA (166 kbp) and a synthetic dialkyl cationic lipid, distearyldimethylammonium bromide (D ₁₈ DAB), was studied using fluorescence microscopy for single-chain observation. The dependence of the higher-order structure of single T4DNAs on the surfactant concentration was evaluated, starting at extremely low values (1.0×10 ^-8 M). We found that individual T4DNA chains undergo a marked discrete transition between elongated coil and compact globule states, and that there is a very wide region of coexistence (about two orders of magnitude of the surfactant concentration) for the coiled and globular T4DNAs. We propose a simple theoretical model for assessing the relationship between the binding equilibrium and the coil–globule transition. In addition, we found that liposomes composed of neutral phospholipids induce unfolding of DNA compacted by a cationic surfactant.